An Internet worm that takes advantage of a known flaw with the Windows (search) operating system raced around the world Monday, snarling tens of thousands of computers and causing Internet traffic to slow.

Though Microsoft Corp. had announced the flaw with the so-called Local Security Authority Subsystem Service (search) last month, many computer owners had yet to apply the fix that the company had released.

Unlike most outbreaks, the Sasser worm (search) does not require users to activate it by clicking on an e-mail attachment. Sasser is known as a network worm because it can automatically scan the Internet for computers with the security flaw and send a copy of itself there.

Security experts said the Sasser worm was spreading rapidly, particularly as businesses reopened Monday but it still was not as widespread as last summer's Blaster outbreak.

They said network operators have gotten more diligent about properly applying the necessary security fixes, and because the worm was first discovered late Friday, many were able to do so over the weekend before employees arrived.

Finland's third largest bank, Sampo, closed 120 of its offices for a few hours on Monday to update antivirus programs. E-banking services and the bank's automated teller machines worked normally.

"We weren't hit by the worm, but we wanted make sure that we wouldn't be affected by it so we closed the branch offices," Sampo spokesman Hannu Vuola said. "The people who suffered the most were those who had actually come to our banks to do business over the counter."